1. Field of Invention
The present invention relates to gas turbines and, in particular, to a fuel-air premixer for a combustor of a gas turbine which uniformly mixes fuel and air so as to reduce Nitrogen Oxide (NOx) formed by the combustion progress.
2. Description of the Related Art
The worldwide concerns of air pollution have led to stricter emission standards requiring significant reduction in gas turbine pollution emission. NOx, which is an inducement to atmospheric pollution, is generally formed in the high temperature regions of the gas turbine combustor by direct oxidation of atmospheric nitrogen with oxygen. Thus reducing the emission of NOx can be achieved by decreasing the temperature of the reaction zone. And one preferred method is to premix fuel and air into a lean mixture prior to combustion. The thermal mass of the excess air absorbs heat and decreases the temperature of the reaction products.
There are several problems associated with dry low emissions combustors operating with lean premixing of fuel and air. Specifically, there is a tendency for flammable mixture of fuel and air within the premixing section of the combustor to combust due to flashback, which occurs when flame propagates from the combustor reaction zone into the premixing section, or auto-ignition, which occurs when the dwell time and temperature of the fuel-air mixture in the premixing section are sufficient for combustion to be initiated without an igniter. The combustion in the premixing section results in degradating of emission performance, and overheating and damaging of the premixing section. Therefore, a problem to be solved is to prevent flashback and auto-ignition within the premixer.
In addition, the fuel and air must be uniformly mixed in the premixer and the reaction zone of the combustor so as to achieve a desired emission performance. In regions in the flow field in which the fuel concentration of the mixture is significantly greater than an average, the temperature of the reaction products in these regions will be higher than an average, and thus a large quantity of thermal NOx will be produced, which makes the combustor fail to meet NOx emission requirements. In regions in the flow field in which the fuel concentration of the mixture is significantly leaner than the average, quenching may occur and oxidizing progress of the hydrocarbons or carbon monoxide may be terminated before reaching equilibrium levels, this can result in failure to meet carbon monoxide (CO) or unburned hydrocarbon (UHC) emission requirements. Thus, another problem to be solved is to mix the fuel and the air with significant uniform concentration distribution in the premixer to meet the emission performance requirements.
Still further, in order to meet emission performance requirements imposed upon the gas turbine in many applications, it is necessary to reduce the fuel concentration of the mixture to a level that is close to the lean flammability limit for most fuels. This results in a reduction in flame propagation speed as well as emissions. As a consequence, lean premixing combustors tend to be less stable than the conventional diffusion flame combustors, and often result in high level combustion driven dynamic pressure activities which often lead to hardware damage, flashback or blowoff. Thus, yet another problem to be solved is to control the combustion driven dynamic pressure activity to an acceptable low level.
Lean, premixing fuel injectors for emission abatement are commonly used in heavy duty industrial gas turbines. A representative example of such a device is described in U.S. Pat. No. 5,259,184. Such devices have achieved great progress in the gas turbine exhaust emission abatement by reducing the NOx emissions by an order of magnitude or more relative to the diffusion flame burners without using diluent injection. The advantages in emission performance, however, have been obtained at the expense of incurring several problems. In particular, flash back and flame holding within the premixer result in degradation of emission performance and hardware damage due to overheating. In addition, the high level combustion driven dynamic pressure activity results in the problems such as flashback, blowoff, and the reduction in the useful life of the combustor hardware.
An example of a method for reducing the combustion driven dynamic pressure activity in lean premixed dry low emissions combustors can be found in U.S. Pat. No. 6,438,961, which proposed an inlet flow conditioner upstream of the premixer inlet to improve the air flow velocity distribution through the premixer and the uniformity of the fuel-air mixture in the premixer, which successfully reduces the premixing flow sensitivity to the air flow mal-distribution in the flow field approaching the premixer.
Though those conventional premixers have achieved progress in premixing fuel and air without introducing some associated problems in premix combustion, much improvement is still needed. The first is to reduce the fuel-air mixing non-uniformity in the premixer which limits the combustors to achieve maximum emission reduction. The second is to resist or prevent the flashback and auto-ignition in the case of various operation conditions and different fuels. The third is to reduce the level of combustion driven dynamic pressure activity so as to obtain high combustion performance in the combustors.